Flash memory device and fabrication process thereof, method of forming a dielectric film

ABSTRACT

A fabrication process of a flash memory device includes microwave excitation of high-density plasma in a mixed gas of Kr and an oxidizing gas or a nitriding gas. The resultant atomic state oxygen O* or hydrogen nitride radicals NH* are used for nitridation or oxidation of a polysilicon electrode surface. It is also disclosed the method of forming an oxide film and a nitride film on a polysilicon film according to such a plasma processing.

TECHNICAL FIELD

[0001] The present invention generally relates to semiconductor devicesand a fabrication process thereof. More particularly, the presentinvention relates to a method of forming a dielectric film andfabrication process of a non-volatile semiconductor memory devicecapable of rewriting information electrically, including a flash memorydevice.

[0002] There are various volatile memory devices such as DRAMs andSRAMs. Further, there are nonvolatile memory devices such as a mask ROM,PROM, EPROM, EEPROM, and the like. Particularly, a flash memory deviceis an EEPROM having a single transistor for one memory cell and has anadvantageous feature of small cell size, large storage capacity and lowpower consumption. Thus, intensive efforts are being made on theimprovement of flash memory devices. In order that a flash memory devicecan be used stably over a long interval of time with low voltage, it isessential to use a uniform insulation film having high film quality.

BACKGROUND ART

[0003] First, the construction of a conventional flash memory devicewill be explained with reference to FIG. 1 showing the concept of agenerally used flash memory device having a so-called stacked gatestructure.

[0004] Referring to FIG. 1, the flash memory device is constructed on asilicon substrate 1700 and includes a source region 1701 and a drainregion 1702 formed in the silicon substrate 1700, a tunneling gate oxidefilm 1703 formed on the silicon substrate 1700 between the source region1701 and the drain region 1702, and a floating gate 1704 formed on thetunneling gate oxide film 1703, wherein there is formed a consecutivestacking of a silicon oxide film 1705, a silicon nitride film 1706 and asilicon oxide film 1707 on the floating gate 1704, and a control gate1708 is formed further on the silicon oxide film 1707. Thus, the flashmemory of such a stacked structure includes a stacked structure in whichthe floating gate 1704 and the control gate 1708 sandwich an insulatingstructure formed of the insulation films 1705, 1706 and 1707therebetween.

[0005] The insulating structure provided between the floating gate 1704and the control gate 1705 is generally formed to have a so-called ONOstructure in which the nitride film 1706 is sandwiched by the oxidefilms 1705 and 1707 for suppressing the leakage current between thefloating gate 1704 and the control gate 1705. In an ordinary flashmemory device, the tunneling gate oxide film 1703 and the silicon oxidefilm 1705 are formed by a thermal oxidation process, while the siliconnitride film 1706 and the silicon oxide film 1707 are formed by a CVDprocess. The silicon oxide film 1705 may be formed by a CVD process. Thetunneling gate oxide film 1703 has a thickness of about 8 nm, while theinsulation films 1705, 1706 and 1707 are formed to have a totalthickness of about 15 nm in terms of oxide equivalent thickness.Further, a low-voltage transistor having a gate oxide film of 3-7 nm inthickness and a high-voltage transistor having a gate oxide film of15-30 nm in thickness are formed on the same silicon in addition to theforegoing memory cell.

[0006] In the flash memory cell having such a stacked structure, avoltage of about 5-7V is applied for example to the drain 1702 whenwriting information together with a high voltage larger than 12V appliedto the control gate 1708. By doing so, the channel hot electrons formedin the vicinity of the drain region 1702 are accumulated in the floatinggate via the tunneling insulation film 1703. When erasing the electronsthus accumulated, the drain region 1702 is made floating and the controlgate 1708 is grounded. Further, a high voltage larger than 12V isapplied to the source region 1701 for pulling out the electronsaccumulated in the floating gate 1704 to the source region 1701.

[0007] Such a conventional flash memory device, on the other hand,requires a high voltage at the time of writing or erasing ofinformation, while the use of such a high voltage tends to cause a largesubstrate current. The large substrate current, in turn, causes theproblem of deterioration of the tunneling insulation film and hence thedegradation of device performance. Further, the use of such a highvoltage limits the number of times rewriting of information can be madein a flash memory device and also causes the problem of erroneouserasing.

[0008] The reason a high voltage has been needed in conventional flashmemory devices is that the ONO film, formed of the insulation films1705, 1706 and 1707, has a large thickness.

[0009] In the conventional art of film formation, there has been aproblem, when a high-temperature process such as thermal oxidationprocess is used in the process of forming an oxide film such as theinsulation film 1705 on the floating gate 1704, in that the quality ofthe interface between the polysilicon gate 1704 and the oxide film tendsto become poor due to the thermal budget effect, etc. In order to avoidthis problem, one may use a low temperature process such as CVD processfor forming the oxide film. However, it has been difficult to form ahigh-quality oxide film according to such a low-temperature process.Because of this reason, conventional flash memory devices had to use alarge thickness for the insulation films 1705, 1706 and 1707 so as tosuppress the leakage current.

[0010] However, the use of large thickness for the insulation films1705, 1706 and 1707 in these conventional flash memory devices hascaused the problem in that it is necessary to use a large writingvoltage and also a large erasing voltage. As a result of using largewriting voltage and large erasing voltage, it has been necessary to formthe tunneling gate insulation film 1703 with large thickness so as toendure the large voltage used.

DISCLOSURE OF THE INVENTION

[0011] Accordingly, it is a general object of the present invention toprovide a novel and useful flash memory device and fabrication processthereof and further a method of forming an insulation film, wherein theforegoing problems are eliminated.

[0012] Another and more specific object of the present invention is toprovide a high-performance flash memory device having a high-qualityinsulation film that is formed at a low temperature process, thethickness of the tunneling gate insulation film or the thickness of theinsulation film between the floating gate and the control gate can bereduced successfully without causing the problem of leakage current, andenabling writing and erasing at low voltage.

[0013] Another object of the present invention is to provide a method offorming an insulation film wherein a high-quality insulation film can beformed on polysilicon.

[0014] Another object of the present invention is to provide a flashmemory device, comprising:

[0015] a silicon substrate,

[0016] a first electrode formed on the silicon substrate with atunneling insulation film interposed therebetween, and

[0017] a second electrode formed on the first electrode with aninsulation film interposed therebetween,

[0018] said insulation film having a stacked structure including atleast one silicon oxide film and one silicon nitride film, at least apart of said silicon oxide film containing Kr with a surface density of10¹⁰ cm⁻² or more.

[0019] According to the present invention, the quality of the insulationfilm used in a flash memory device between a floating gate electrode anda control gate electrode is improved by forming the insulation film byan oxidation reaction or nitriding reaction conducted in Ar or Kr plasmain which atomic state oxygen O* or hydrogen nitride radicals NH* areformed efficiently. Further, it becomes possible to reduce the thicknessof the insulation film without causing unwanted increase of leakagecurrent. As a result, the flash memory device of the present inventioncan operate at high speed with low voltage and has a long lifetime.

[0020] Another object of the present invention is to provide a method offabricating a flash memory device comprising a silicon substrate, afirst electrode of polysilicon formed on the silicon substrate with aninsulation film interposed therebetween, and a second electrode formedon the first electrode with an inter-electrode insulation filminterposed therebetween, said inter-electrode insulation film having astacked structure containing at least one silicon oxide film and onesilicon nitride film,

[0021] said silicon oxide film being formed by the step of exposing asilicon oxide film deposited by a CVD process to atomic state oxygen O*formed by microwave excitation of plasma in a mixed gas of anoxygen-containing gas and an inert gas predominantly of a Kr gas.

[0022] According to the present invention, an oxide film havingexcellent leakage current characteristic is obtained for theinter-electrode insulation film, and it becomes possible to form a flashmemory having a simple structure, capable of holding electric charges inthe floating gate electrode stably, and is operable at a low drivingvoltage.

[0023] Another object of the present invention is to provide afabrication process of a flash memory device comprising a siliconsubstrate, a first electrode of polysilicon formed on the siliconsubstrate with an insulation film interposed therebetween, and a secondelectrode formed on the first electrode with an inter-electrodeinsulation film interposed therebetween, said inter-electrode insulationfilm having a stacked structure including at least one silicon oxidefilm and one silicon nitride film,

[0024] said silicon nitride film being formed by exposing a siliconnitride film deposited by a CVD process to hydrogen nitride radicals NH*formed by microwave excitation of plasma in a mixed gas of an NH₃ gas oralternatively a gas containing N₂ and H₂ and a gas predominantly formedof an Ar or Kr gas.

[0025] According to the present invention, a nitride film havingexcellent leakage current characteristic suitable for theinter-electrode insulation film is obtained. Thus, it becomes possibleto realize a flash memory having a simple construction and is capable ofholding electric charges stably in the floating gate electrode. Theflash memory thus obtained is operable at a low driving voltage.

[0026] Another object of the present invention is to provide a method offorming a silicon oxide film, comprising the steps of:

[0027] depositing a polysilicon film on a substrate; and

[0028] forming a silicon oxide film on a surface of said polysiliconfilm by exposing the surface of said polysilicon film to atomic stateoxygen O* formed by microwave excitation of plasma in a mixed gas of agas containing oxygen and an inert gas predominantly of a Kr gas.

[0029] According to the present invention, it becomes possible to form ahomogeneous silicon oxide film on a polysilicon film with uniformthickness irrespective of the orientation of the silicon crystalstherein. The silicon oxide film thus formed has excellent leakagecurrent characteristic comparative to that of a thermal oxide film andcauses a Fowler-Nordheim tunneling similarly to the case of a thermaloxide film.

[0030] Another object of the present invention is to provide a method offorming a silicon nitride film, comprising the steps of:

[0031] depositing a polysilicon film on a substrate; and

[0032] forming a nitride film on a surface of said polysilicon film byexposing the surface of said polysilicon film to hydrogen nitrideradicals NH* formed by microwave excitation of plasma in a mixed gas ofa gas containing nitrogen and hydrogen as constituent elements and aninert gas predominantly of an Ar gas or a Kr gas.

[0033] According to the present invention, it becomes possible to form anitride film of excellent characteristic on the surface of a polysiliconfilm.

[0034] Another object of the present invention is to provide a method offorming a dielectric film, comprising the steps of:

[0035] depositing a polysilicon film on a substrate; and

[0036] converting a surface of said polysilicon film into a dielectricfilm by exposing said polysilicon film to a microwave-excited plasmaformed in a mixed gas of an inert gas predominantly of Ar or Kr and agas containing oxygen as a constituent element and a gas containingnitrogen as a constituent element.

[0037] According to the present invention, it becomes possible to forman oxynitride film having excellent characteristic on the surface of apolysilicon film.

[0038] Another object of the present invention is to provide a method offabricating a flash memory having a silicon substrate, a first electrodeof polysilicon formed on said silicon substrate with an insulation filminterposed therebetween, and a second electrode formed on said firstelectrode with an inter-electrode oxide film interposed therebetween,said inter-electrode oxide film being formed by the steps of:

[0039] depositing a polysilicon film on said silicon substrate as saidfirst electrode; and

[0040] exposing a surface of said polysilicon film to atomic stateoxygen O* formed by microwave excitation of plasma in a mixed gas of agas containing oxygen and an inert gas predominantly of a Kr gas.

[0041] According to the present invention, an oxide film havingexcellent leakage current characteristic is obtained for theinter-electrode insulation film, and it becomes possible to realize aflash memory having a simple construction and is capable of holdingelectric charges in the floating gate electrode stably. The flash memorythus formed is operable at a low driving voltage.

[0042] Another object of the present invention is to provide a method offabricating a flash memory having a silicon substrate, a first electrodeof polysilicon formed on said silicon substrate with an oxide filminterposed therebetween, and a second electrode of polysilicon formed onsaid first electrode with an inter-electrode nitride film interposedtherebetween, said inter-electrode nitride film being formed by thesteps of:

[0043] depositing a polysilicon film on said silicon substrate as saidfirst electrode; and

[0044] exposing a surface of said polysilicon film to hydrogen nitrideradicals NH* formed by microwave excitation of plasma in a mixed gas ofa gas containing nitrogen and hydrogen and an inert gas predominantly ofan Ar gas or a Kr gas.

[0045] According to the present invention, a nitride film havingexcellent leakage current characteristic is obtained for theinter-electrode nitride film and it becomes possible to realize a flashmemory having a simple construction and is capable of holding electriccharges in the floating gate electrode stably. The flash memory thusformed is operable at a low driving voltage.

[0046] Another object of the present invention is to provide a method offabricating a flash memory having a silicon substrate, a first electrodeof polysilicon formed on said silicon substrate with insulation filminterposed therebetween, and a second electrode of polysilicon formed onsaid first electrode with an inter-electrode oxynitride film interposedtherebetween, said inter-electrode oxynitride film being formed by thesteps of:

[0047] depositing a polysilicon film on said silicon substrate as saidfirst electrode; and

[0048] converting a surface of said polysilicon film into a siliconoxynitride film by exposing said polysilicon film to microwave excitedplasma formed in a mixed gas of an inert gas predominantly of Ar or Krand a gas containing oxygen and nitrogen.

[0049] According to the present invention, an oxynitride film havingexcellent leakage current characteristic is obtained for theinter-electrode insulation film, and it becomes possible to realize aflash memory capable of holding electric charges stably in the floatinggate electrode. The flash memory thus formed is operable at a lowdriving voltage.

[0050] Another object of the present invention is to provide a method offorming a silicon oxide film on a polysilicon film, comprising the stepsof:

[0051] forming atomic state oxygen O* in a processing vessel of amicrowave processing apparatus, said microwave processing apparatusincluding: a shower plate in a part of said processing vessel such thatsaid shower plate extends parallel to a substrate to be processed, saidshower plate having a plurality of apertures for supplying a plasma gastoward said substrate; and a microwave radiation antenna emitting amicrowave into said processing vessel via said shower plate, bysupplying a gas predominantly of Kr and a gas containing oxygen intosaid processing vessel via said shower plate and further by supplyingsaid microwave into said processing vessel from said microwave radiationantenna through said shower plate; and

[0052] forming a silicon oxide film by causing oxidation in a surface ofa polysilicon film formed on said substrate by said plasma in saidprocessing vessel.

[0053] According to the present invention, atomic state oxygen thatcause oxidation in a polysilicon film are formed efficiently by inducinghigh-density plasma of low electron temperature in the processingchamber as a result of microwave excitation of the plasma gas supplieduniformly from the shower plate. The silicon oxide film thus formed bythe Kr plasma is irrelevant to the crystal orientation of the Sicrystals on which the silicon oxide film is formed. Thus, the siliconoxide film is formed uniformly on the polysilicon film. The siliconoxide film contains little surface states and is characterized by smallleakage current. According to the present invention, the oxidationprocessing of the polysilicon film can be conducted at a low temperatureof 550° C. or less, and there occurs no substantial grain growth in thepolysilicon film even when such an oxidation process is conducted. Thus,the problem of concentration of electric field, and the like, whicharises with such a grain growth is avoided.

[0054] Another object of the present invention is to provide a method offorming a silicon nitride film on a polysilicon film, said methodcomprising the steps of:

[0055] forming plasma containing hydrogen nitride radicals NH* in aprocessing vessel of a microwave processing apparatus, said microwaveprocessing apparatus including: a shower plate in a part of saidprocessing vessel so as to extend parallel to a substrate to beprocessed, said shower plate having a plurality of apertures forsupplying a plasma gas to said substrate; and a microwave radiationantenna emitting a microwave into said processing vessel via said showerplate, by supplying a gas predominantly of Ar or Kr and a gas containingnitrogen and hydrogen into said processing vessel from said shower plateand by further supplying said microwave into said processing vessel fromsaid microwave radiation antenna through said shower plate; and

[0056] forming a silicon nitride film by nitriding a surface of apolysilicon film formed on said substrate by said plasma in saidprocessing vessel.

[0057] According to the present invention, hydrogen nitride radicals NH*that cause nitridation in the polysilicon film are formed efficiently byinducing high-density plasma having a low electron temperature in theprocessing chamber by microwave excitation of the plasma gas supplieduniformly from the shower plate. The silicon nitride film thus formed bythe Kr plasma has an advantageous feature of small leakage current inspite of the fact that the silicon nitride film is formed at a lowtemperature.

[0058] Another object of the present invention is to provide a method offabricating a flash memory device, said flash memory device having asilicon substrate and including a first electrode formed on said siliconsubstrate with a tunneling insulation film interposed therebetween and asecond electrode formed on said first electrode with an insulation filminterposed therebetween, said insulation film having a stacked structurecontaining at least one silicon oxide film and one silicon nitride film,said silicon oxide film being formed by the steps of:

[0059] introducing a gas containing oxygen and a gas predominantly of aKr gas into a processing chamber, and causing microwave excitation ofplasma in said processing chamber.

[0060] According to the present invention, it becomes possible tooxidize the surface of the first electrode at low temperature, byconducting the oxidation processing in the Kr plasma in which atomicstate oxygen O* are formed efficiently. As a result, an oxide filmcontaining small surface states and is characterized by small leakagecurrent can be obtained for the desired silicon oxide film.

[0061] Another object of the present invention is to provide afabrication process of a flash memory device having a silicon substrate,a first electrode formed on said silicon substrate with a tunnelinginsulation film interposed therebetween, and a second electrode formedon said first electrode with an insulation film interposed therebetween,said insulation film having a stacked structure containing at least onesilicon oxide film and one silicon nitride film,

[0062] said silicon nitride film being formed by introducing an NH₃ gasor a gas containing N₂ and H₂ and a gas predominantly of Ar or Kr into aprocessing chamber, and causing microwave excitation of plasma in saidprocessing chamber.

[0063] According to the present invention, it becomes possible tonitride the surface of the first electrode at low temperature byconducting the nitridation in the plasma of Ar or Kr in which thehydrogen nitride radicals NH* are formed efficiently.

[0064] Other objects and further features of the present invention willbecome apparent from the following detailed description of the inventionwhen read in conjunction with the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0065]FIG. 1 is a cross-sectional diagram showing a schematiccross-section of a conventional flash memory device;

[0066]FIG. 2 is a diagram showing the concept of the plasma apparatusthat uses a radial line slot antenna:

[0067]FIG. 3 is a diagram showing the relationship between a thicknessand a gas pressure in a processing chamber for an oxide film formedaccording to a first embodiment of the present invention;

[0068]FIG. 4 is a diagram showing the relationship between the thicknessand duration of oxidation for the oxide film formed according to thefirst embodiment of the present invention;

[0069]FIG. 5 is a diagram showing the depth profile of Kr density in thesilicon oxide film according to the first embodiment of the presentinvention;

[0070]FIG. 6 is a diagram showing the surface state density in thesilicon oxide film according to the first embodiment of the presentinvention;

[0071]FIG. 7 is a diagram showing the relationship between the surfacestate density and the breakdown voltage for the silicon oxide filmaccording to the first embodiment of the present invention;

[0072]FIGS. 8A and 8B are diagrams showing the relationship between thesurface state density and break down voltage of the silicon oxide filmobtained according to the first embodiment of the present invention andthe total pressure of the processing chamber;

[0073]FIG. 9 is a diagram showing the dependence of film thickness onthe total pressure used in the processing chamber for a nitride filmformed according to a second embodiment of the present invention;

[0074]FIG. 10 is a diagram showing the current-voltage characteristic ofthe silicon nitride film according to the second embodiment of thepresent invention;

[0075]FIGS. 11A and 11B are diagrams showing the oxidation process,nitriding process and oxy-nitriding process of a polysilicon filmaccording to a third embodiment of the present invention;

[0076]FIG. 12 is a diagram showing the dependence of film thickness onthe oxidation duration for an oxide film obtained by an oxidationprocessing of a polysilicon film according to a third embodiment of thepresent invention;

[0077] FIGS. 13A-13C are diagrams showing a change of surface morphologyassociated with the oxidation process of a polysilicon film according tothe third embodiment of the present invention;

[0078]FIGS. 14A and 14B are diagrams showing a change of surfacemorphology of a polysilicon film when subjected to a thermal oxidationprocess;

[0079]FIGS. 15A and 15B are diagrams showing the transmission electronmicroscope image of a polysilicon film formed according to the thirdembodiment of the present invention;

[0080] FIGS. 16-17 are diagrams showing the electric properties of theoxide film formed on a polysilicon according to the third embodiment ofthe present invention in comparison with a thermal oxide film;

[0081]FIG. 18 is a diagram showing the cross-sectional structure of aflash memory device according to a fourth embodiment of the presentinvention;

[0082]FIG. 19 is a diagram showing the cross-sectional structure of aflash memory device according to a fifth embodiment of the presentinvention;

[0083] FIGS. 20-23 are diagrams showing the fabrication process of aflash memory device according to a fifth embodiment of the presentinvention;

[0084]FIG. 24 is a diagram showing the cross-sectional structure of theflash memory device according to a sixth embodiment of the presentinvention; and

[0085]FIG. 25 is a diagram showing the cross-sectional structure of aflash memory device according to a seventh embodiment of the presentinvention.

BEST MODE FOR IMPLEMENTING THE INVENTION

[0086] Hereinafter, embodiments of the present invention will bedescribed.

First Embodiment

[0087] First, low temperature oxide film formation using plasma will bedescribed.

[0088]FIG. 2 is a cross sectional diagram showing the construction of anexemplary microwave plasma processing apparatus used in the presentinvention for realizing the oxidation process, wherein the microwaveplasma processing apparatus uses a radial line slot antenna (seeWO98/33362). The novel feature of the present embodiment is to use Kr asthe plasma excitation gas at the time of forming the oxide film.

[0089] Referring to FIG. 2, the microwave plasma processing apparatusincludes a vacuum vessel (processing chamber) 101 accommodating thereina stage 104 on which a substrate 103 to be processed is supported. Theprocessing chamber 101 is evacuated to a vacuum state, and a Kr gas andan O₂ gas are introduced from a shower plate 102 formed at a part of thewall of the processing chamber 101 such the pressure inside theprocessing chamber is set to about 1 Torr (about 133 Pa). Further, adisk-shaped substrate such as a silicon wafer is placed on the stage 104as the foregoing substrate 103. The stage 104 includes a heatingmechanism, and the temperature of the substrate 103 is set to about 400°C. It is preferable to set the temperature in the range of 200-550° C.As long as the temperature is set in this range, a similar result isobtained.

[0090] Next, a microwave of 2.45 GHz is supplied from an externalmicrowave source via a coaxial waveguide 105 connected thereto, whereinthe microwave thus supplied is radiated into the processing chamber 101by the radial line slot antenna 106 through a dielectric plate 107. As aresult, there is formed high-density plasma in the processing chamber101. As long as the frequency of the microwave is in the range of 900MHz or more but not exceeding 10 GHz, a similar result is obtained asdescribed below. In the illustrated example, the distance between theshower plate 102 and the substrate 103 is set to about 6 cm. Narrowerthe distance, faster the film forming process.

[0091] In the microwave plasma processing apparatus of FIG. 2, itbecomes possible to realize a plasma density exceeding 1×10¹² cm⁻³ atthe surface of the substrate 103. Further, the high-density plasma thusformed by microwave excitation has a low electron temperature, and aplasma potential of 10V or less is realized at the surface of thesubstrate 103. Thus, the problem of the substrate 103 being damaged bythe plasma is positively eliminated. Further, there occurs no problem ofcontamination of the substrate 103 because of the absence of plasmasputtering in the processing chamber 101. Because of the fact that theplasma processing is conducted in a narrow space between the showerplate 102 and the substrate 103, the product material of the reactionflows quickly in the lateral direction to a large volume spacesurrounding the stage 104 and is evacuated. Thereby, a very uniformprocessing is realized.

[0092] In the high-density plasma in which an Kr gas and an O₂ gas aremixed, Kr* at the intermediate excitation state cause collision with theO₂ molecules and there occurs efficient formation of atomic state oxygenO*, and the atomic state oxygen O* thus formed cause oxidation of thesubstrate surface. It should be noted that oxidation of a siliconsurface has conventionally been conducted by using H₂O or O₂ moleculesat very high process temperature such as 800° C. or more. In the case ofusing atomic state oxygen, on the other hand, it becomes possible tocarry out the oxidation process at a low temperature of 550° C. or less.

[0093] In order to increase the chance of collision between K* and O₂,it is preferable to increase the pressure in the processing chamber 101.On the other hand, the use of too high pressure in the processingchamber increases the chance that O* causing collision with another O*and returning to the O₂ molecule. Thus, there would exist an optimum gaspressure.

[0094]FIG. 3 shows the thickness of the oxide film for the case in whichthe total pressure inside the processing chamber 101 is changed whilemaintaining the Kr and oxygen pressure ratio such that the proportion ofKr is 97% and the proportion of oxygen is 3%. In the experiment of FIG.3, it should be noted that the silicon substrate was held at 400° C. andthe oxidation was conducted over the duration of 10 minutes.

[0095] Referring to FIG. 3, it can be seen that the thickness of theoxide film becomes maximum when the total gas pressure in the processingchamber 101 is set to 1 Torr, indicating that the oxidation processbecomes optimum under this pressure or in the vicinity of this pressure.Further, it should be noted that this optimum pressure remains the samein the case the silicon substrate has the (100) oriented surface andalso in the case the silicon substrate has the (111) oriented surface.

[0096]FIG. 4 shows the relationship between the thickness of the oxidefilm and the duration of the oxidation processing for the oxide filmthat is formed by oxidation of the silicon substrate surface using theKr/O₂ high-density plasma. In FIG. 4, the result for the case in whichthe silicon substrate has the (100) oriented surface and the result forthe case in which the silicon substrate has the (111) oriented surfaceare both represented. Further, FIG. 4 also represents the oxidation timedependence for the case a conventional dry oxidation process at thetemperature of 900° C. is employed.

[0097] Referring to FIG. 4, it can be seen that the oxidation ratecaused by the Kr/O₂ high-density plasma oxidation processing, conductedat the temperature of 400° C. under the chamber pressure of 1 Torr, islarger than the oxidation rate for a dry O₂ process conducted at 900° C.under the atmospheric pressure.

[0098] In the case of conventional dry thermal oxidation process at 900°C., it can be seen that the growth rate of the oxidation film is largerwhen the oxide film is formed on the (111) oriented silicon surface ascompared with the case of forming the oxide film on the (100) orientedsilicon surface. In the case in which the Kr/O₂ high-density plasmaoxidation process is used, on the other hand, this relationship isreversed and the growth rate of the oxide film on the (111) surface issmaller than the growth rate of the oxide film on the (100) surface. Inview of the fact that silicon atoms are arranged with larger,surfacedensity on the (111) oriented surface than on the (100) oriented surfacein a Si substrate, it is predicted that the oxidation rate should besmaller on the (111) surface than on the (100) surface as long as thesupply rate of the oxygen radicals is the same. The result of theforegoing oxidation process of the silicon substrate surface is in goodconformity with this prediction when the Kr/O₂ high-density plasma isused for the oxidation process, indicating that there is formed a denseoxide film similar to the one formed on a (100) surface, also on the(111) surface. In the conventional case, on the other hand, theoxidation rate of the (111) surface is much larger than the oxidationrate of the (100) surface. This indicates that the oxide film formed onthe (111) film would be sparse in quality as compared with the oxidefilm formed on the (100) surface.

[0099]FIG. 5 shows the depth profile of the Kr density inside thesilicon oxide film that is formed according to the foregoing process,wherein the depth profile FIG. 5 was obtained by a total-reflectionfluorescent X-ray spectrometer. In the experiment of FIG. 5, theformation of the silicon oxide film was conducted at the substratetemperature of 400° C. while setting the oxygen partial pressure in theKr gas to 3% and setting the pressure of the processing chamber to 1Torr (about 133 Pa).

[0100] Referring to FIG. 5, it can be seen that the surface density ofKr decreases toward the silicon/silicon oxide interface, and a densityof 2×10¹¹ cm⁻² is observed at the surface of the silicon oxide film.Thus, the result of FIG. 5 indicates that a substantially uniform Krconcentration is realized in the silicon oxide film when the siliconoxide film is formed by surface oxidation of a silicon substrate whileusing the Kr/O₂ high-density plasma, provided that the silicon oxidefilm has a thickness of 4 nm or more. It can be seen that the Krconcentration in the silicon oxide film decreases toward thesilicon/silicon oxide surface. According to the method of silicon oxideformation of the present invention, Kr is incorporated in the siliconoxide film with a surface density of 10¹⁰ cm⁻² or more. The result ofFIG. 5 is obtained on the (100) surface and also on the (111) surface.

[0101]FIG. 6 shows the surface state density formed in an oxide film,wherein the result of FIG. 6 was obtained by a low-frequency C-Vmeasurement. The silicon oxide film of FIG. 6 was formed at thesubstrate temperature of 400° C. while using the apparatus of FIG. 2. Inthe experiment, the oxygen partial pressure in the rare gas was set to3% and the pressure in the processing chamber was set to 1 Torr (about133 Pa). For the sake of comparison, the surface state density of athermal oxide film formed at 900° C. in a 100% oxygen atmosphere is alsorepresented.

[0102] Referring to FIG. 6, it can be seen that the surface statedensity of the oxide film is small in both of the cases in which theoxide film is formed on the (100) surface and in which the oxide film isformed on the (111) surface as long as the oxide film is formed whileusing the Kr gas. The value of the surface state density thus achievedis comparable with the surface state density of a thermal oxide filmthat is formed on the (100) surface in a dry oxidation atmosphere at900° C. Contrary to the foregoing, the thermal oxide film formed on the(111) surface has a surface state density larger than the foregoingsurface state density by a factor of 10.

[0103] The mechanism of the foregoing results is thought as follows.

[0104] Viewing the silicon crystal from the side of the silicon oxidefilm, there appear two bonds for one silicon atom when the siliconsurface is the (100) surface. On the other hand, there appear one bondand three bonds alternately for one silicon atom when the siliconsurface is the (111) surface. Thus, when a conventional thermaloxidation process is applied to a (111) surface, oxygen atoms quicklycause bonding to all the foregoing three bonds, leaving the remainingbond behind the silicon atom. Thereby, the remaining bond may extend andform a weak bond or disconnected and form a dangling bond. When this isthe case, there inevitably occurs an increase of surface state density.

[0105] When the high-density plasma oxidation is conducted in the mixedgas of Kr and O₂, Kr* of the intermediate excitation state causecollision with O₂ molecules and there occurs efficient formation ofatomic state oxygen O*, wherein the atomic state oxygen O* thus formedeasily reach the weak bond or dangling bond noted before and form a newsilicon-oxygen bond. With this, it is believed that the surface statesare reduced also on the (111) surface.

[0106] In the experiment for measuring the relationship between theoxygen partial pressure in the Kr gas used for the atmosphere during theformation of the silicon oxide film and the breakdown voltage of thesilicon insulation film thus formed, and further in the experiment formeasuring the relationship between the oxygen partial pressure in the Krgas and the surface state density in the silicon oxide film thus formed,it was confirmed that a generally same result is obtained for the casein which the silicon oxide film is formed on the (100) surface and forthe case in which the silicon oxide film is formed on the (111) surface,and that the surface state density becomes minimum when the oxygenpartial pressure in the Kr gas is set to 3%, provided that the siliconoxide film is formed by setting the pressure of the processing chamberto 1 Torr (about 133 Pa). Further, the breakdown voltage of the siliconoxide film becomes maximum when the oxygen partial pressure is set toabout 3%. From the foregoing, it is derived that an oxygen partialpressure of 2-4% is preferable for conducting the oxidation process byusing the Kr/O₂ mixed gas.

[0107]FIG. 7 shows a relationship between the pressure used for formingthe silicon oxide film and the breakdown voltage of the silicon oxidefilm thus formed. Further, FIG. 7 shows the relationship between thepressure and the surface state density of the silicon oxide film. InFIG. 7, it should be noted that the oxygen partial pressure is set to3%.

[0108] Referring to FIG. 7, it can be seen that the breakdown voltage ofthe silicon oxide film becomes maximum and the surface state densitybecomes minimum when the pressure of about 1 Torr is used at the time offorming the oxide film. From the result of FIG. 7, it is concluded thatthe preferable pressure of forming an oxide film by using a Kr/O₂ mixedgas would be 800-1200 mTorr. The result of FIG. 7 is valid not only forthe process on the (100) surface but also for the process on the (111)surface.

[0109] In addition to the foregoing, other various preferablecharacteristics were obtained for the oxide film formed by the oxidationof silicon substrate surface by the Kr/O₂ high-density plasma withregard to electronic and reliability characteristics, including thebreakdown characteristic, the leakage characteristic, the hotcarrierresistance, and the QBD (Charge-to-Breakdown) characteristic, whichrepresents the amount of electric charges that leads a silicon oxidefilm to breakdown as a result of application of a stress current,wherein the characteristics thus obtained are comparable to those of thethermal oxide film that is formed at 900° C.

[0110]FIGS. 8A and 8B show the leakage current induced by a stresscurrent for a silicon oxide film thus obtained, in comparison with thecase of a conventional thermal oxide film. In FIGS. 8A and 8B, thethermal oxide film has a thickness of 3.2 nm.

[0111] Referring to FIGS. 8A and 8B, it can be seen that there occurs anincrease of leakage current with injection of electric charges into theconventional thermal oxide film, while there occurs no such a change ofelectric current in the plasma oxide film that is formed by using theKr/O₂ plasma, even in the case electric charges of 100 C/cm² areinjected. Thus, the silicon oxide film of the present invention has avery long lifetime and it takes a very long time for a tunneling currentto cause degradation in the oxide film. The oxide film of the presentinvention is thus most suitable for the tunneling oxide film of a flashmemory device.

[0112] As noted previously, the oxide film grown by the Kr/O₂high-density plasma has a characteristic comparable with, or superiorto, the conventional high-temperature thermal oxide film formed on the(100) surface, for both of cases in which the oxide film is grown on the(100) surface and the oxide film is grown on the (111) surface, in spiteof the fact that the oxide film is formed at a low temperature of 400°C. It is noted that the existence of Kr in the oxide film contributesalso to this effect. More specifically, the existence of Kr in the oxidefilm causes relaxation of stress at the Si/SiO₂ interface and decreaseof the electric charges in the film and the surface state density,leading to remarkable improvement of electric properties of the oxidefilm. Particularly, the existence of Kr atoms with a density of 10¹⁰cm⁻² as represented in FIG. 5 is believed to contribute to theimprovement of electric properties and reliability properties of thesilicon oxide film.

Second Embodiment

[0113] Next, the process of forming a nitride film at a low temperatureby using high-density microwave plasma will be described.

[0114] In the formation of the nitride film, the same apparatus as theone explained with reference to FIG. 2 is used, except that Ar or Kr isused for the plasma excitation gas at the time of forming the nitridefilm.

[0115] Thus, the vacuum vessel (processing chamber) 101 is evacuated toa high vacuum state first, and the pressure inside the processingchamber 101 is then set to about 100 mTorr (about 13 Pa) by introducingan Ar gas and a NH₃ gas via the shower plate 102, and the like. Further,a disk-shaped substrate such as a silicon wafer is placed on the stage104 as the substrate 103 and the substrate temperature is set to about500° C. As long as the substrate temperature is in the range of 400-500°C., almost the same results are obtained.

[0116] Next, a microwave of 2.45 GHz is introduced into the processingchamber from the coaxial waveguide 105 via the radial line slot antenna106 and further through the dielectric plate 107, and there is inducedhigh-density plasma in the processing chamber. It should be noted that asimilar result is obtained as long as a microwave in the frequency of900 MHz or more but not exceeding 10 GHz is used. In the illustratedexample, the distance between the shower plate 102 and the substrate 103is set to 6 cm. Narrower the distance, faster the film formation rate.While the present embodiment shows the example of forming a film byusing the plasma apparatus that uses the radial line slot antenna, it ispossible to use other method for introducing the microwave into theprocessing chamber.

[0117] In the present embodiment, it should be noted that an Ar gas isused for exciting plasma. However, a similar result is obtained alsowhen a Kr gas is used. While the present embodiment uses NH₃ for theplasma process gas, it is also possible to use a mixed gas of N₂ and H₂for this purpose.

[0118] In the high-density plasma excited in the mixed gas of Ar or Krand NH₃ (or alternatively N₂ and H₂), there are formed NH* radicalsefficiently by Ar* or Kr* having an intermediate excitation state, andthe NH* radicals thus formed cause the desired nitridation of thesubstrate surface. Conventionally, there has been no report of directnitridation of silicon surface. Thus, a nitride film has been formed bya plasma CVD process, and the like. However, the nitride film thusformed by a conventional plasma CVD process does not have the qualityrequired for a gate insulation film of a transistor. In the nitridationof silicon according to the present embodiment, on the other hand, it ispossible to form a high-quality nitride film at low temperature on anyof the (100) surface and the (111) surface, irrespective of the surfaceorientation of the silicon substrate.

[0119] Meanwhile, it should be noted that existence of hydrogen is animportant factor when forming a silicon nitride film. With the existenceof hydrogen in plasma, the dangling bonds existing in the siliconnitride film or at the nitride film interface are terminated in the formof Si—H bond or N—H bond, and the problem of electron trapping withinthe silicon nitride film or on the silicon nitride interface iseliminated. The existence of the Si—H bond and the N—H bond in thenitride film is confirmed in the present invention by infraredabsorption spectroscopy or X-ray photoelectron spectroscopy. As a resultof the existence of hydrogen, the hysteresis of the CV characteristic isalso eliminated. Further, it is possible to suppress the surface statedensity of the silicon/silicon nitride interface below 3×10¹⁰ cm⁻² bysetting the substrate temperature to 500° C. or more. In the event thesilicon nitride film is formed by using an inert gas (Ar or Kr) and amixed gas of N₂/H₂, the number of the traps of electrons or holes in thefilm decreases sharply by setting the partial pressure of the hydrogengas to 0.5% or more.

[0120]FIG. 9 shows the pressure dependence of the film thickness of thesilicon nitride film thus formed according to the foregoing process. Inthe illustrated example, the ratio of the Ar gas to the NH₃ gas is setto 98:2 in terms of partial pressure, and the film formation wasconducted over the duration of 30 minutes.

[0121] Referring to FIG. 9, it can be seen that the growth rate of thenitride film increases when the pressure in the processing chamber 101is reduced so as to increase the energy given to NH₃ (or N₂/H₂) from theinert gas (Ar or Kr). From the viewpoint of efficiency of nitridation,it is therefore preferable to use the gas pressure of 50-100 mTorr(about 7-13 Pa). Further, it is preferable to set the partial pressureof NH₃ (or N₂/H₂) in the rare gas atmosphere to 1-10%, more preferablyto 2-6%.

[0122] It should be noted that the silicon nitride film of the presentembodiment has a dielectric constant of 7.9, which is almost twice aslarge as that of a silicon oxide film.

[0123]FIG. 10 shows the current-voltage characteristic of the siliconnitride film of the present embodiment. It should be noted that theresult of FIG. 10 is obtained for the case in which a silicon nitridefilm having a thickness of 4.2 nm (2.1 nm in terms of oxide filmequivalent thickness) is formed by using a gas mixture of Ar/N₂/H₂ whilesetting the gas composition ratio, Ar:N₂:H₂, to 93:5:2 in terms ofpartial pressure. In FIG. 10, the result for the foregoing nitride filmis compared also with the case of a thermal oxide film having athickness of 2.1 nm.

[0124] Referring to FIG. 10, it can be seen that there is realized avery small leakage current, smaller than the leakage current of asilicon oxide film by a factor of 10⁴ or more, is obtained when avoltage of 1V is applied for the measurement. This result indicates thatthe silicon nitride film thus obtained can be used as the insulatingfilm that is provided between a floating gate electrode and a controlgate electrode of a flash memory device for suppressing the leakagecurrent flowing therebetween.

[0125] It should be noted that the foregoing condition of filmformation, the property of the film, or the electric characteristic ofthe film are obtained similarly on any of the surfaces of the siliconcrystal. In other words, the same result is obtained on the (100)surface and also on the (111) surface. According to the presentinvention, therefore, it is possible to form a silicon nitride film ofexcellent quality on any of the crystal surfaces of silicon. It shouldbe noted that the existence of the Si—H bond or N—H bond in the film isnot the only cause of the foregoing advantageous effect of the presentinvention. The existence of Ar or Kr in the film contributes also to theforegoing advantageous result. As a result of the existence of Ar or Krin the film, it should be noted that the stress within the nitride filmor the stress at the silicon/nitride film interface is relaxedsubstantially, while this relaxation of stress also contributes to thereduction of fixed electric charges and the surface state density in thesilicon nitride film, which leads to the remarkable improvement ofelectric properties and reliability. Particularly, the existence of Aror Kr with the density of 10¹⁰ cm⁻² is thought as contributingeffectively to the improvement of electric characteristics andreliability of the silicon nitride film, just in the case of the siliconoxide film represented in FIG. 5.

Third Embodiment

[0126] The foregoing method of forming oxide film or nitride film isapplicable also to the oxidation or nitridation of polysilicon. Thus,the present invention enables formation of a high-quality oxide film ornitride film on polysilicon.

[0127] Hereinafter, the method of forming a dielectric film on apolysilicon film according to a third embodiment of the presentinvention will be described with reference to FIGS. 11A and 11B.

[0128] Referring to FIG. 11A, a polysilicon film 203 is deposited on asilicon substrate 201 covered by an insulation film 202. By exposing thepolysilicon film 203 to the high-density mixed gas plasma of Kr or Arand oxygen in the processing vessel 101 of the microwave plasmaprocessing apparatus of FIG. 2 in the step of FIG. 11B, a silicon oxidefilm 204 having a high film quality is obtained on the surface of thepolysilicon film 203, wherein the silicon oxide film 204 thus formed ischaracterized by small surface state density and small leakage current.

[0129] In the step of FIG. 11B, it is also possible to form ahigh-quality nitride film 205 on the surface of the polysilicon film 203by exposing the polysilicon film 203 to the high-density mixed gasplasma of Kr or Ar and NH₃ or N₂ and H₂.

[0130] Further, it is possible, in the step of FIG. 11B, to form ahigh-quality oxynitride film 206 on the surface of the polysilicon film203, by exposing the polysilicon film 203 to the high-density mixed gasplasma of Kr or Ar and oxygen and NH₃ or N₂ and H₂.

[0131] It should be noted that a polysilicon film formed on aninsulation film tends to take a stable state in which the (111) surfaceis oriented in the direction perpendicular to the insulation film. Thepolysilicon film having this state is dense and provides good quality.On the other hand, crystal grains of other crystal orientation may existalso in the polysilicon film. According to the method of forming anoxide film or a nitride film or an oxynitride film of the presentembodiment, it becomes possible to form a high-quality oxide film, or ahigh-quality nitride film or a high-quality oxynitride film,irrespective of the surface orientation of silicon layer. Thus, theprocess of FIGS. 11A and 11B is most suitable for forming a high qualitythin oxide film or a nitride film or an oxynitride film on a polysiliconfilm. It should be noted that the polysilicon film may be the firstpolysilicon gate electrode that constitutes the floating electrode offlash memory. As the oxide film or nitride film or oxynitride film ofthe present invention can be formed at a low temperature of 550° C. orless, there arises no problem of rough surface formation on thepolysilicon surface.

[0132]FIG. 12 shows the result of the experiment of forming an oxidefilm on an n-type polysilicon film having the thickness of 200 nm,wherein it should be noted that the polysilicon film is formed on athermal oxide film covering the (100) oriented surface of a Si substratewith a thickness of 100 nm. It should be noted that FIG. 12 also showsthe case in which the (100) surface and the (111) surface of a Sisubstrate is oxidized directly. In FIG. 12, the vertical axis representsthe thickness of the oxide film thus formed, while the horizontal axisrepresents the duration of the process. Further, ▴ in FIG. 12 shows thecase in which an oxide film is formed by processing the polysiliconsurface thus formed by the Kr/O₂ plasma, while  in FIG. 12 shows thecase in which an oxide film is formed by processing the (100) surface ofthe Si substrate by the Kr/O₂ plasma. Further, ▪ in FIG. 12 shows thecase in which an oxide film is formed by processing the (111) surface ofthe Si substrate by the Kr/O₂ plasma. In FIG. 12, it should further benoted that ∘ represents the case of causing thermal oxidation of the(100) surface of the Si substrate, while □ represents the case ofcausing a thermal oxidation of the (111) surface of the Si substrate.Further, Δ represents the case in which thermal oxidation is applied tothe surface of a polysilicon film. It should be noted that the Kr/O₂plasma processing was conducted at the temperature of 400° C., by usingthe apparatus explained already with reference to FIG. 2 while settingthe internal pressure of the processing chamber 101 to 1 Torr (about 133Pa) and setting the ratio of the Kr gas and the oxygen gas to 97:3 interms of flow-rate. On the other hand, the thermal oxidation process wasconducted at 900° C. in the 100% oxygen atmosphere. In the experiment ofFIG. 12, it should be noted that the polysilicon film is doped to acarrier density exceeding 10² cm⁻³.

[0133] Referring to FIG. 12, no substantial difference of oxidationprocess can be seen when the oxidation process is conducted on the (100)surface and when the oxidation process is conducted on the (111)surface, as long as the Kr/O₂ plasma process is used for the oxidationprocess, as explained already. Further, it can be seen thatsubstantially the same oxidation rate is achieved in the case ofoxidizing the polysilicon film. Further, it should be noted that theoxidation rate thus obtained is substantially identical with theoxidation rate observed when applying a thermal oxidation process to apolysilicon film. In contrast, it can be seen that, when theconventional thermal oxidation process is applied, the oxidation rate ofthe Si substrate surface is much slower, indicating that the oxide filmthus formed has a much smaller thickness.

[0134] From FIG. 12, it will be understood that a nearly identicaloxidation rate is achieved for a Si surface as long as the Kr/O₂ plasmais used for the oxidation process, irrespective of whether the Sisurface is a surface of a single-crystal Si of an arbitrary orientationor a polycrystalline surface including grain boundaries.

[0135]FIG. 13A shows the result of atomic-force. microscopy applied tothe surface of a polysilicon film thus formed before the oxidationprocess is conducted.

[0136]FIG. 13B, on the other hand, shows the state of the polysiliconsurface of FIG. 13A after the Kr/O₂ plasma processing is conducted. Inthe state of FIG. 13B, it should be noted that the polysilicon surfaceis covered by the oxide film formed as a result of the Kr/O₂ plasmaprocess. Further, FIG. 13C shows the polysilicon surface in the statethe oxide film is removed from the surface of FIG. 13B by an HFprocessing.

[0137] Referring to FIGS. 13A-13C, the oxidation process in the Kr/O₂plasma is effective at low temperature as low as 400° C., and there iscaused no substantial grain growth in the polysilicon film. Associatedtherewith, there is no problem of surface roughening in the polysiliconfilm. The oxide film thus has a generally uniform thickness.

[0138] In contrast, FIG. 14A shows the surface state of a polysiliconfilm subjected to thermal oxidation process at 900° C. in the state thatthe polysilicon film carries thereon the oxide film, while FIG. 14Bshows the surface state in which the oxide film of FIG. 14A is removed.

[0139] Referring to FIGS. 14A and 14B, it can be seen that there occursa substantial crystal grain growth in the polysilicon film as a resultof the thermal processing, and that there has been caused a substantialroughening in the polysilicon film surface. When a thin oxide film isformed on such a rough surface, there tends to occur the problem ofconcentration of electric field, while such a concentration of theelectric field causes the problem in the leakage current characteristicsand problems in the breakdown characteristics.

[0140]FIGS. 15A and 15B represent the result of transmission microscopicobservation showing the cross-section of the specimen in which an oxidefilm is formed on a polysilicon film surface by the Kr/O₂ plasmaprocessing. It should be noted that FIG. 15B shows a part of the area ofFIG. 15A in an enlarged scale.

[0141] Referring to FIG. 15A, it can be seen that there is formed an Allayer on the oxide film (designated as “polyoxide”), wherein FIG. 15Aclearly shows that the oxide film thus formed has a uniform thickness onthe polysilicon film surface. Further, the enlarged view of FIG. 15Bindicates that the oxide film is uniform.

[0142]FIG. 16 shows the relationship between the current density of thesilicon oxide film thus formed on the polysilicon film and the electricfield applied thereto, in comparison with a corresponding relationshipfor a thermal oxide film. Further, FIG. 17 is a diagram that shows therelationship of FIG. 16 in the Fowler-Nordheim plot.

[0143] Referring to FIGS. 16 and 17, it can be seen that the tunnelingcurrent starts to increase in the case the oxide film is formed on thepolysilicon film by the Kr/O₂ plasma oxidation process when the appliedelectric field has exceeded 5 MV/cm. Further, the plot of FIG. 17indicates that the tunneling current flowing through the oxide film is aFowler-Nordheim tunneling current, similarly to the case of the thermaloxide film. Further, from FIG. 17, it can be seen that there appears alarger barrier height φ_(B) of tunneling in the case the oxide film isformed by the Kr/O₂ plasma oxidation process as compared with the caseof the thermal oxide film. Further, it can be seen that there is causedan increase of breakdown voltage as compared with the case ofconventional thermal oxide film.

Fourth Embodiment

[0144] Next, the construction of a flash memory device according to afourth embodiment of the present invention will be described withreference to FIG. 18, wherein the flash memory device of the presentembodiment uses the art of the low-temperature oxide film formationconducted in the microwave plasma explained before.

[0145] Referring to FIG. 18, the flash memory device is constructed on asilicon substrate 1001 and includes a tunneling oxide film 1002 formedon the silicon substrate 1001 and a first polysilicon gate electrode1003 formed on the tunneling oxide film 1002 as a floating gateelectrode, wherein the polysilicon gate electrode 1003 is covered by asilicon oxide film 1004, and a second polysilicon gate electrode 1008 isformed on the silicon oxide film 1004 as a control gate electrode. InFIG. 18, illustration of source region, drain region, contact holes,interconnection patterns, and the like is omitted.

[0146] In the flash memory device of such a construction, a high qualityfilm characterized by small leakage current is obtained for the oxidefilm 1004 as a result of the exposure of the polysilicon gate electrode1003 to the high-density plasma that is formed in the microwave plasmaprocessing apparatus of FIG. 2 by using the Kr/O₂ plasma gas. Thus, itbecomes possible to reduce the thickness of the oxide film 1004, andlow-voltage driving of the flash memory device becomes possible.

[0147] In the flash memory device of FIG. 18, it is also possible to usea nitride film 1005 formed by the Kr/NH₃ plasma processing as explainedbefore in place of the oxide film 1004. Further, it is also possible touse an oxynitride film 1009 as explained before with reference to theprevious embodiment.

Fifth Embodiment

[0148] Next, the fabrication process of a flash memory device accordingto a fifth embodiment of the present invention will be described,wherein the flash memory device of the present embodiment uses thetechnology of low-temperature formation of oxide film and nitride filmwhile using the microwave plasma explained above, wherein the presentembodiment also includes a high-voltage transistor and a low-voltagetransistor having a gate electrode of polysilicon/silicide stackedstructure.

[0149]FIG. 19 shows the schematic cross-sectional structure of a flashmemory device 1000 according to the present embodiment.

[0150] Referring to FIG. 19, the flash memory device 1000 is constructedon the silicon substrate 1001 and includes the tunneling oxide film 1002formed on the silicon substrate 1001 and the first polysilicon gateelectrode 1003 formed on the tunneling oxide film 1002 as a floatinggate electrode, wherein the polysilicon gate electrode 1003 is furthercovered consecutively by the silicon nitride film 1004, a silicon oxidefilm 1005, a silicon nitride film 1006 and a silicon oxide film 1007,and the second polysilicon gate electrode 1008 is formed further on thesilicon nitride film 1007 as a control gate electrode. In FIG. 19,illustration of source region, drain region, contact holes,interconnection patterns, and the like, is omitted.

[0151] In the flash memory of the present embodiment, the silicon oxidefilms 1002, 1005 and 1007 are formed according to the process of siliconoxide film formation explained before. Further, the silicon nitridefilms 1004 and 1006 are formed according to the process of siliconnitride film formation explained before. Thus, excellent electricproperty is guaranteed even when the thickness of these films is reducedto one-half the thickness of conventional oxide film or nitride film.

[0152] Next, the fabrication process of a semiconductor integratedcircuit including the flash memory device of the present embodiment willbe explained with reference to FIGS. 20-25.

[0153] Referring to FIG. 20, a silicon substrate 1101 carries a fieldoxide film 1102 such that the field oxide film 1102 defines, on thesilicon substrate 1101, a flash memory cell region A, a high-voltagetransistor region B and a low-voltage transistor region C, wherein eachof the regions A-C is formed with a silicon oxide film 1103. The fieldoxide film 1102 may be formed by a selective oxidation (LOCOS) processor a shallow trench isolation process.

[0154] In the present embodiment, a Kr gas is used for the plasmaexcitation gas at the time of formation of the oxide film and thenitride film. Further, the microwave plasma processing apparatus of FIG.2 is used for the formation of the oxide film and the nitride film.

[0155] Next, in the step of FIG. 21, the silicon oxide film 1103 isremoved in the memory cell region A and a tunneling oxide film 1104 isformed on the memory cell region A with a thickness of about 5 nm.During the formation of the tunneling oxide film 1104, the vacuum vessel(reaction chamber) 101 is evacuated to a vacuum state and the Kr gas andan O₂ gas is introduced from the shower plate 102 such that the pressureinside of the reaction chamber reaches 1 Torr (about 133 Pa). Further,the temperature of the silicon wafer is set to 450° C., and a microwaveof 2.56 GHz frequency in the coaxial waveguide 105 is supplied to theinterior of the processing chamber via the radial line slot antenna 106and the dielectric plate 107. As a result, there is formed ahigh-density plasma.

[0156] In the step of FIG. 21, a first polysilicon film 1105 isdeposited, after the step of forming the tunneling oxide film 1104, suchthat the first polysilicon film 1105 covers the tunneling oxide film1104, and the surface of the polysilicon film 1105 thus deposited isplanarized by conducting a hydrogen radical processing. Further, thefirst polysilicon film 1105 is removed from the high-voltage transistorregion B and the low-voltage transistor region by way of patterning,leaving the first polysilicon film 1105 selectively on the tunnelingoxide film 1104 of the memory cell region.

[0157] Next, in the step of FIG. 22, a lower nitride film 1106A, a loweroxide film 1106B, an upper nitride film 1106C and an upper oxide film1106D are formed consecutively on the structure of FIG. 21. As a result,an insulation film 1106 having an NONO structure is formed by using themicrowave plasma processing apparatus of FIG. 2.

[0158] In more detail, the vacuum vessel (processing chamber) 101 of themicrowave plasma processing apparatus of FIG. 2 is evacuated to ahigh-vacuum state, and the Kr gas, an N₂ gas and an H₂ gas areintroduced into the processing chamber 101 from the shower plate 102until the pressure inside the processing chamber is set to about 100mTorr (about 13 Pa). Further, the temperature of the silicon wafer isset to 500° C. In this state, a microwave of 2.45 GHz frequency isintroduced into the processing chamber from the coaxial waveguide 105via the radial line slot antenna 106 and the dielectric plate 107, andthere is formed a high-density plasma in the processing chamber. As aresult of this, a silicon nitride film of about 6 nm thickness is formedon the polysilicon surface as the lower nitride film 1106A.

[0159] Next, the supply of the microwave is interrupted. Further, thesupply of the Kr gas, the N₂ gas and the H₂ gas is interrupted, and thevacuum vessel (processing chamber) 101 is evacuated. Thereafter, the Krgas and the O₂ gas are introduced again into the processing chamber viathe shower plate 102, and the pressure in the processing chamber is setto 1 Torr (about 133 Pa). In this state, the microwave of 2.45 GHzfrequency is supplied again, and there is formed high-density plasma inthe processing chamber 101. As a result, a silicon oxide film of about 2nm thickness is formed as the lower oxide film 1106B.

[0160] Next, the supply of the microwave is again interrupted. Further,the supply of the Kr gas and the O₂ gas is interrupted, and theprocessing chamber 101 is evacuated. Thereafter, the Kr gas, the N₂ gasand the H₂ gas are introduced into the processing chamber via the showerplate 102 so that the pressure inside the processing chamber is set to100 mTorr (about 13 Pa). In this state, a microwave of 2.45 GHzfrequency is introduced and high-density plasma is formed in theprocessing chamber 101. As a result of the plasma processing using thehigh-density plasma thus formed, there is further formed a siliconnitride film of 3 nm thickness.

[0161] Finally, the supply of the microwave is interrupted. Further, thesupply of the Kr gas, the N₂ gas and the H₂ gas is also interrupted, andthe vacuum vessel (processing chamber) 101 is evacuated. Thereafter, theKr gas and the O₂ gas are introduced again via the shower plate 102 andthe pressure inside the processing chamber is set to 1 Torr (about 133Pa). In this state, the microwave of 2.45 GHz frequency is againsupplied, and high-density plasma is formed in the processing chamber101. As a result, a silicon oxide film of 2 nm thickness is formed asthe upper oxide film 1106D.

[0162] Thus, according to the foregoing process steps, it becomespossible to form the insulation film 1106 of the NONO structure with athickness of 9 nm. It was confirmed that the NONO film 1106 thus formeddoes not depends on the surface orientation of polysilicon and that eachof the oxide films and the nitride films therein is highly uniform interms of film thickness and film quality.

[0163] In the step of FIG. 22, the insulation film 1106 thus formed isfurther subjected to a patterning process such that the insulation film1106 is selectively removed in the high-voltage transistor region B andin the low-voltage transistor region C.

[0164] Next, in the step of FIG. 23, an ion implantation process isconducted into the high-voltage transistor region B and further into thelow-voltage transistor region C for the purpose of threshold control.Thereafter, the oxide film 1103 is removed from the foregoing regions Band C, and a gate oxide film 1107 is formed on the high-voltagetransistor region B with a thickness of 7 nm, followed by the formationof a gate oxide film 1108 on the low-voltage transistor region C with athickness of 3.5 nm.

[0165] In the step of FIG. 23, the overall structure including the fieldoxide film 1102 is covered consecutively with a second polysilicon film1109 and a silicide film 1110. By patterning the polysilicon film 1109and the silicide film 1110, a gate electrode 111B is formed in thehigh-voltage transistor region B and a gate electrode 111C is formed inthe low-voltage transistor region C. Further, the polysilicon film 1109and the silicide film 110 are patterned in the memory cell region, and agate electrode 1111A is formed.

[0166] Finally, a standard semiconductor process including formation ofsource and drain regions, formation of insulation films, formation ofcontact holes and formation of interconnections, is conducted, and thesemiconductor device is completed.

[0167] It should be noted that the silicon oxide film and the siliconnitride film in the NONO film 1101 thus formed shows excellent electricproperties in spite of the fact that the each of the silicon oxide andsilicon nitride films therein has a very small thickness. Further, thesilicon oxide film and the silicon nitride film are dense and have afeature of high film quality. As the silicon oxide film and the siliconnitride film are formed at low temperature, there occurs no problem ofthermal budget formation, and the like, at the interface between thegate polysilicon and the oxide film, and an excellent interface isobtained.

[0168] In the flash memory integrated circuit device in which the flashmemory devices of the present invention are arranged in atwo-dimensional array, it becomes possible to carry out writing anderasing of information at low voltage. Further, the semiconductorintegrated circuit has advantageous features of suppressing substratecurrent and suppressing degradation of the tunneling insulation film.Thus, the semiconductor integrated circuit has a reliable devicecharacteristic. The flash memory device of the present invention ischaracterized by a low leakage current, and enables writing ofinformation at a voltage of about 7V. Further, the flash memory deviceof the present invention can retain the written information over aduration longer than a conventional flash memory device by a factor of10. The number of times the rewriting can be made is increased also by afactor of 10 in the case of the flash memory of the present inventionover a conventional flash memory device.

Sixth Embodiment

[0169] Next, a flash memory device according to a second embodiment ofthe present invention will be described, wherein the flash memory deviceof the present embodiment has a gate electrode having apolysilicon/silicide stacked structure and is formed by using the art oflow-temperature formation of oxide and nitride film that uses thehigh-density microwave plasma explained before.

[0170]FIG. 24 shows a schematic cross-sectional structure of a flashmemory device 1500 according to the present embodiment.

[0171] Referring to FIG. 24, the flash memory device 1500 is constructedon a silicon substrate 1501 and includes a tunneling nitride film 1502formed on the silicon substrate 1501 and a first polysilicon gateelectrode 1503 formed on the tunneling nitride film 1502 as a floatinggate electrode, wherein the first polysilicon gate electrode 1503 iscovered consecutively by a silicon oxide film 1504, a silicon nitridefilm 1505 and a silicon oxide film 1506. Further, a second polysiliconelectrode 1507 forming a control gate electrode is formed on the siliconoxide film 1506. In FIG. 24, illustration of source region, drainregion, contact holes, interconnection patterns, and the like, isomitted.

[0172] In the flash memory device 1500 of FIG. 24, the silicon oxidefilms 1502, 1504 and 1506 are formed according to a process of forming asilicon oxide film that uses the high-density microwave plasma explainedbefore. Further, the silicon nitride film 1505 is formed by a process offorming a silicon nitride film that uses the high-density microwaveplasma explained before.

[0173] In the present embodiment, too, the process steps up to the stepof patterning the first polysilicon film 1503 are identical with thoseof the steps of FIGS. 20 and 21, except for the point that the tunnelingnitride film 1502 is formed after the step of evacuating the vacuumvessel (processing chamber) 101, by introducing an Ar gas, an N₂ gas andan H₂ gas from the shower plate 102 such that the pressure inside theprocessing chamber becomes 100 mTorr (about 13 Pa). Thereby, thetunneling nitride film 1502 is formed to have a thickness of about 4 nm,by supplying a microwave of 2.45 GHz to form high-density plasma in theprocessing chamber.

[0174] After the first polysilicon film 1503 is thus formed, the lowersilicon oxide film 1504 and the silicon nitride film 1505 and the uppersilicon oxide film 1506 are formed consecutively on the firstpolysilicon film, and an insulation film having an ONO structure isobtained.

[0175] In more detail, the vacuum chamber (processing chamber) 101 ofthe microwave plasma processing apparatus explained previously withreference to FIG. 2 is evacuated to a high vacuum state, and the Kr gasand an O₂ gas are introduced into the processing chamber via the showerplate 102 such that the pressure of the processing chamber 101 is set to1 Torr (about 133 Pa). In this state, the microwave of 2.45 GHz issupplied to the processing chamber 101 and there is formed thehigh-density plasma therein. As a result, a silicon oxide film having athickness of about 2 nm is formed on the surface of the firstpolysilicon film 1503.

[0176] Next, a silicon nitride film is formed on the silicon oxide filmby a CVD process with a thickness of 3 nm, and the vacuum vessel(processing chamber) 101 is evacuated. Further, the Ar gas, the N₂ gasand the H₂ gas are introduced into the processing chamber via the showerplate 102, and the pressure inside the processing chamber is set to 1Torr (about 133 Pa). In this state, the microwave of 2.45 GHz issupplied again and the high-density plasma is formed in the processingchamber 101. By exposing the foregoing silicon nitride film to thehydrogen nitride radicals NH* formed with the high-density plasma, thesilicon nitride film is converted to a dense silicon nitride film.

[0177] Next, a silicon oxide film is formed on the foregoing densesilicon nitride film by a CVD process with a thickness of about 2 nm,and the pressure of the processing chamber 101 of the microwave plasmaprocessing apparatus is set to 1 Torr (about 133 Pa) by supplyingthereto the Kr gas and the O₂ gas. By supplying the microwave of 2.45GHz further to the processing chamber in this state, the high-densityplasma is formed in the processing chamber 101. Thereby, the CVD oxidefilm formed previously in the CVD process is converted to a densesilicon oxide film by exposing to the atomic state oxygen O* formed withthe high-density plasma.

[0178] Thus, an ONO film is formed on the polysilicon film 1503 with athickness of about 7 nm. The ONO film thus formed shows no dependence ofproperty thereon on the orientation of the polysilicon surface on whichthe ONO film is formed and has an extremely uniform thickness. The ONOfilm thus formed is then subjected to a patterning process for removinga part thereof corresponding to the high-voltage transistor region B andthe low-voltage transistor region C. By further applying the processsteps similar to those used in the fourth embodiment before, the devicefabrication process is completed.

[0179] The flash memory device thus formed has an excellent leakagecharacteristic characterized by low leakage current, and writing andreading operation can be conducted at the voltage of about 6V. Further,the flash memory device provides a memory retention time larger by thefactor of 10 over the conventional flash memory devices, similarly tothe flash memory device 1000 of the previous embodiment. Further, it ispossible to achieve the number of rewriting operations larger by thefactor of 10 over the conventional flash memory devices.

Seventh Embodiment

[0180] Next, a description will be made on a flash memory device 1600according to a seventh embodiment of the present invention, wherein theflash memory device 1600 has a gate electrode of polysilicon/silicidestacked structure and is formed by the process that uses the microwavehigh-density plasma for forming low temperature oxide and nitride films.

[0181]FIG. 25 shows the schematic cross-sectional structure of the flashmemory device 1600.

[0182] Referring to FIG. 25, the flash memory device 1600 is constructedon a silicon substrate 1601 and includes a tunneling oxide film 1602formed on the silicon substrate 1061 and a first polysilicon gateelectrode 1603 formed on the tunneling oxide film 1602, wherein thefirst polysilicon gate electrode 1603 is covered consecutively by asilicon nitride film 1604 and a silicon oxide film 1605. Further, asecond polysilicon gate electrode 1606 is formed on the silicon oxidefilm 1605 as a control gate electrode.

[0183] In FIG. 25, illustration of source region, drain region, contactholes, and interconnection patterns, is omitted.

[0184] In the flash memory 1600 of FIG. 25, the silicon oxide films 1602and 1605 are formed by the film forming process of oxide film explainedabove, while the silicon nitride film 1604 is formed by the film formingprocess of nitride film also explained above.

[0185] Next, the fabrication process of a flash memory integratedcircuit according to the present invention will be explained.

[0186] In the present embodiment, too, the process proceeds similarly tothe previous embodiments up to the step of patterning the firstpolysilicon film 1603, and the first polysilicon film 1603 is formed inthe region A. Thereafter, an insulation film having an NO structure isformed by consecutively depositing a silicon nitride film and a silic0onoxide film on the first polysilicon film 1603.

[0187] In more detail, the NO film is formed by using the microwaveplasma processing apparatus of FIG. 2 according to the process stepsnoted below.

[0188] First, the vacuum vessel (processing chamber) 101 is evacuated,and a Kr gas, an N2 gas and an H2 gas are introduced thereto via theshower plate 102 and the pressure inside the processing chamber is setto about 100 mTorr (about 13 Pa). In this state, a microwave of 2.45 GHzis supplied, and high-density plasma is induced in the processingchamber. Thereby, there occurs a nitriding reaction in the polysiliconfilm 1603 and a silicon nitride film is formed with a thickness of about3 nm.

[0189] Next, a silicon oxide film is formed by a CVD process to athickness of about 2 nm, and a Kr gas and an O₂ gas are introduced inthe microwave plasma processing apparatus such that the pressure insidethe processing chamber is set to about 1 Torr (about 133 Pa). In thissate, a microwave of 2.45 GHz frequency is supplied to form high-densityplasma in the processing chamber, such that the oxide film formed by theCVD process is exposed to the atomic state oxygen O* associated with thehigh-density plasma. As a result, the CVD oxide film is converted to adense silicon oxide film.

[0190] The NO film is thus formed to a thickness of about 5 nm, whereinthe NO film thus formed has an extremely uniform thickness irrespectiveof the surface orientation of the polysilicon crystals. The NO film thusformed is then subjected to a patterning process and the part thereofcovering the high-voltage transistor region B and the low-voltagetransistor region C are removed selectively.

[0191] After the foregoing process, the process steps similar to thoseof FIG. 23 are conducted and the device fabrication process iscompleted.

[0192] It should be noted that the flash memory device thus formed has alow leakage characteristic, and enables writing or erasing at a lowvoltage as low as 5V. Further, the flash memory device provides a memoryretention time larger than the conventional memory retention time by afactor of 10, and rewriting cycles larger than the conventionalrewriting cycles by a factor of 10.

[0193] It should be noted that the fabrication process of the memorycell, the high-voltage transistor and the low-voltage transistor merelyrepresents an example, and the present invention is by no means limitedto these embodiments. For example, it is possible to use an Ar gas inplace of the Kr gas during the formation process of the nitride film.Further, it is possible to use a film having a stacked structure ofpolysilicon/silicide/polysilicon/refractory metal/amorphous silicon orpolysilicon, for the first and second polysilicon films.

[0194] Further, it is also possible to use another plasma processingapparatus in place of the microwave plasma processing apparatus of FIG.2 for forming the oxide film or nitride film of the present invention,as long as the plasma processing apparatus enables low temperatureformation of an oxide film. Further, the radial line slot antenna is notthe only solution for introducing a microwave into the processingchamber of the plasma processing apparatus, and the microwave may beintroduced by other means.

[0195] In place of the microwave plasma processing apparatus of FIG. 2,it is also possible to use a plasma processing apparatus having atwo-stage shower plate construction, in which the plasma gas such as theKr gas or Ar gas is introduced from a first shower plate and theprocessing gas is introduced from a second shower plate different fromthe first shower plate. In this case, it is also possible to introducethe oxygen gas from the second shower plate. Further, it is possible todesign the process such that the floating gate electrode of the flashmemory device and the gate electrode of the high-voltage transistor areformed simultaneously by the first polysilicon electrode.

[0196] Further, the present invention is not limited to the embodimentsdescribed heretofore, but various variations and modifications may bemade without departing from the scope of the invention.

Industrial Applicability

[0197] According to the present invention, it becomes possible to form ahigh-quality silicon oxide film, silicon nitride film or siliconoxynitride film on a polysilicon film with excellent characteristics andreliability comparable with, or superior to, those of a silicon thermaloxide film formed at a high temperature of about 1000° C. or a CVDsilicon nitride film, by using a Kr-containing insulation film formed bya novel plasma oxidation process or nitridation process conducted at alow temperature lower than 550° C. Thus, the present invention realizesa high quality and high-performance flash memory device, which allowsrewriting operation at low voltage and provides excellent electriccharge retention characteristic.

1. A flash memory device, characterized by: a silicon substrate, a firstelectrode formed on said silicon substrate with an insulation filminterposed therebetween, and a second electrode formed on said firstelectrode with an inter-electrode insulation film interposedtherebetween, said inter-electrode insulation film having a stackedstructure including at least one silicon oxide film and one siliconnitride film, at least a part of said silicon oxide film containing Krwith a surface density of 10¹⁰ cm⁻² or more.
 2. A flash memory device asclaimed in claim 1, characterized in that said first electrode includesa polysilicon film on a surface thereof, and wherein saidinter-electrode insulation film has a stacked structure in which a firstsilicon nitride film, a first silicon oxide film, a second siliconnitride film and a second silicon oxide film are stacked consecutively.3. A flash memory device as claimed in claim 1, characterized in thatsaid first electrode includes a polysilicon film on a surface thereof,and wherein said inter-electrode insulation film is formed of threelayers of a silicon oxide film, a silicon nitride film and a siliconoxide film.
 4. A flash memory device as claimed in claim 1,characterized in that said first electrode includes a polysilicon filmon a surface thereof, and wherein said inter-electrode film is formed oftwo layers of a first silicon nitride film and a second silicon oxidefilm.
 5. A method of fabricating a flash memory device, said flashmemory device comprising a silicon substrate, a first electrode formedon said silicon substrate with an insulation film interposedtherebetween, and a second electrode formed on said first electrode withan inter-electrode insulation film interposed therebetween, saidinter-electrode insulation film having a stacked structure includingtherein at least one silicon oxide film and one silicon nitride film,characterized in that said silicon oxide film is formed by a processcomprising the steps of: supplying a gas containing oxygen and a gaspredominantly of Kr into a processing chamber, and exciting plasma insaid processing chamber by a microwave.
 6. A method of fabricating aflash memory device, said flash memory device comprising a siliconsubstrate, a first electrode formed on said silicon substrate with aninsulation film interposed therebetween, and a second electrode formedon said first electrode with an inter-electrode insulation filminterposed therebetween, said inter-electrode insulation film having astacked structure in which a first silicon nitride film, a first siliconoxide film, a second silicon nitride film and a second silicon oxidefilm are stacked consecutively, said first electrode having apolysilicon surface, characterized in that said first and second siliconoxide films are formed by a process comprising the steps of: introducinga gas containing oxygen and a gas predominantly of Kr into a processingchamber, and exciting plasma in said processing chamber by a microwave.7. A method of fabricating a flash memory device, said flash memorydevice comprising a silicon substrate, a first electrode formed on saidsilicon substrate with an insulation film interposed therebetween, and asecond electrode formed on said first electrode with an inter-electrodeinsulation film interposed therebetween, said inter-electrode insulationfilm having a stacked structure in which a first silicon oxide film, asilicon nitride film and a second silicon oxide film are stackedconsecutively, said first electrode having a polysilicon surface,characterized in that said first and second silicon oxide films areformed by a process comprising the steps of: introducing a gascontaining oxygen and a gas predominantly of Kr into a processingchamber, and exciting plasma in said processing chamber by a microwave.8. A method of fabricating a flash memory device, said flash memorydevice comprising a silicon substrate, a first electrode formed on saidsilicon substrate with an insulation film interposed therebetween, and asecond electrode formed on said first electrode with an inter-electrodeinsulation film interposed therebetween, said inter-electrode insulationfilm having a two-layer structure in which a silicon oxide film and asilicon nitride film are stacked consecutively, said first electrodehaving a polysilicon surface, characterized in that said silicon oxidefilm are formed by a process comprising the steps of: introducing a gascontaining oxygen and a gas predominantly of Kr into a processingchamber, and exciting plasma in said processing chamber by a microwave.9. A method of fabricating a flash memory device, said flash memorydevice comprising a silicon substrate, a first electrode formed on saidsilicon substrate with an insulation film interposed therebetween, and asecond electrode formed on said first electrode with an inter-electrodeinsulation film interposed therebetween, said inter-electrode insulationfilm having a stacked structure including at least one silicon oxidefilm and at least one silicon nitride film, characterized in that saidsilicon oxide film is formed by a process comprising the step of:exposing a silicon oxide film deposited by a CVD process to atomic stateoxygen O* formed by microwave excitation of plasma in a mixed gas of anoxygen-containing gas and an inert gas predominantly of a Kr gas.
 10. Afabrication process of a flash memory device, said flash memory devicecomprising a silicon substrate, a first electrode formed on said siliconsubstrate with an insulation film interposed therebetween, and a secondelectrode formed on said first electrode with an inter-electrodeinsulation film interposed therebetween, said inter-electrode insulationfilm having a stacked structure in which a first silicon nitride film, afirst silicon oxide film, a second silicon nitride film and a secondsilicon oxide film are stacked consecutively, said first electrodehaving a polysilicon surface, characterized in that said first andsecond silicon oxide films are formed by a process comprising the stepof: exposing a silicon oxide film deposited by a CVD process to atomicstate oxygen O* formed by exciting plasma in a mixed gas of a gascontaining oxygen and a gas predominantly of a Kr gas, by a microwave.11. A method of fabricating a flash memory device, said flash memorydevice comprising a silicon substrate, a first electrode formed on saidsilicon substrate with an insulation film interposed therebetween, and asecond electrode formed on said first electrode with an inter-electrodeinsulation film interposed therebetween, said inter-electrode insulationfilm having a stacked structure in which a first silicon oxide film, asilicon nitride film and a second silicon oxide film are stackedconsecutively, said first electrode having a polysilicon surface,characterized in that said second silicon oxide film are formed by aprocess comprising the step of: exposing a silicon oxide film depositedby a CVD process to atomic state oxygen O* formed by exciting plasma ina mixed gas of a gas containing oxygen and a gas predominantly of a Krgas by a microwave.
 12. A method of fabricating a flash memory device,said flash memory device comprising a silicon substrate, a firstelectrode formed on said silicon substrate with an insulation filminterposed therebetween, and a second electrode formed on said firstelectrode with an inter-electrode insulation film interposedtherebetween, said inter-electrode insulation film having a stackedstructure including at least one silicon oxide film and at least onesilicon nitride film, characterized in that said silicon nitride filmare formed by a process comprising the steps of: introducing a gascontaining any of an NH₃ gas or an N₂ gas and an H₂ gas and a gaspredominantly of an Ar gas or a Kr gas into a processing chamber, andexciting plasma in said processing chamber by a microwave.
 13. A methodof fabricating a flash memory device, said flash memory devicecomprising a silicon substrate, a first electrode formed on said siliconsubstrate with an insulation film interposed therebetween, and a secondelectrode formed on said first electrode with an inter-electrodeinsulation film interposed therebetween, said inter-electrode insulationfilm having a stacked structure in which a first silicon nitride film, afirst silicon oxide film, a second silicon nitride film and a secondsilicon oxide film are stacked consecutively, said first electrodehaving a polysilicon surface, characterized in that said first andsecond silicon nitride films are formed by a process comprising thesteps of: introducing an NH₃ gas or a gas containing N₂ and H₂ and a gaspredominantly of an Ar gas or a Kr gas into a processing chamber, andexciting plasma in said processing chamber by a microwave.
 14. A methodof fabricating a flash memory device, said flash memory devicecomprising a silicon substrate, a first electrode formed on said siliconsubstrate with an insulation film interposed therebetween, and a secondelectrode formed on said first electrode with an inter-electrodeinsulation film interposed therebetween, said inter-electrode insulationfilm having a stacked structure in which a first silicon oxide film, asilicon nitride film and a second silicon oxide film are stackedconsecutively, said first electrode having a polysilicon surface,characterized in that said silicon oxide film are formed by a processcomprising the steps of: introducing an NH₃ gas or a gas containing N₂and H₂ and a gas predominantly of an Ar gas or a Kr gas into aprocessing chamber, and exciting plasma in said processing chamber by amicrowave.
 15. A method of fabricating a flash memory device, said flashmemory device comprising a silicon substrate, a first electrode formedon said silicon substrate with an insulation film interposedtherebetween, and a second electrode formed on said first electrode withan inter-electrode insulation film interposed therebetween, saidinter-electrode insulation film having a two-layer structure in which asilicon oxide film and a silicon nitride film are stacked consecutively,said first electrode having a polysilicon surface, characterized in thatsaid silicon nitride film are formed by a process comprising the stepsof: introducing an NH₃ gas or a gas containing N₂ and H₂ and a gaspredominantly of an Ar gas or a Kr gas into a processing chamber, andexciting plasma in said processing chamber by a microwave.
 16. A methodof fabricating a flash memory device, said flash memory devicecomprising a silicon substrate, a first electrode formed on said siliconsubstrate with an insulation film interposed therebetween, and a secondelectrode formed on said first electrode with an inter-electrodeinsulation film interposed therebetween, said inter-electrode insulationfilm having a stacked structure containing at least one silicon oxidefilm and at least one silicon nitride film, characterized in that saidsilicon nitride film is formed by a process comprising the step of:exposing a silicon nitride film deposited by a CVD process to hydrogennitride radicals NH* formed by microwave excitation of plasma in a mixedgas of an NH₃ gas or a gas containing N₂ and H₂ and a gas predominantlyof an Ar gas or a Kr gas.
 17. A method of fabricating a flash memorydevice, said flash memory device comprising a silicon substrate, a firstelectrode formed on said silicon substrate with an insulation filminterposed therebetween, and a second electrode formed on said firstelectrode with an inter-electrode insulation film interposedtherebetween, said inter-electrode insulation film having a stackedstructure in which a first silicon nitride film, a first silicon oxidefilm, a second silicon nitride film and a second silicon oxide film arestacked consecutively, said first electrode having a polysiliconsurface, characterized in that each of said first and second siliconnitride films is formed by a process comprising the step of: exposing asilicon nitride film deposited by a CVD process to hydrogen nitrideradicals NH* formed by exciting plasma in a mixed gas of an NH₃ gas or agas containing N₂ and H₂ and a gas predominantly of an Ar gas or a Krgas by a microwave.
 18. A method of fabricating a flash memory device,said flash memory device comprising a silicon substrate, a firstelectrode formed on said silicon substrate with an insulation filminterposed therebetween, and a second electrode formed on said firstelectrode with an inter-electrode insulation film interposedtherebetween, said first electrode having a polysilicon surface,characterized in that said silicon nitride film is formed by a processcomprising the step of: exposing a silicon nitride film deposited by aCVD process to hydrogen nitride radicals NH* formed by exciting plasmain a mixed gas of an NH₃ gas or a gas containing N₂ and H₂ and a gaspredominantly of an Ar gas or a Kr gas by a microwave.
 19. A method offabricating a flash memory device, said flash memory device comprising asilicon substrate, a first electrode formed on said silicon substratewith an insulation film interposed therebetween, and a second electrodeformed on said first electrode with an inter-electrode insulation filminterposed therebetween, said inter-electrode insulation film having atwo-layer structure in which a silicon oxide film and a silicon nitridefilm are stacked consecutively, said first electrode having apolysilicon surface, characterized in that said inter-electrodeinsulation film is formed by a process comprising the step of: exposinga silicon nitride film deposited by a CVD profess to hydrogen nitrideradicals NH* formed by exciting plasma in a mixed gas of an NH₃ gas or agas containing N₂ and H₂ and a gas predominantly of an Ar gas or a Krgas by a microwave.
 20. A method of fabricating a flash memory device,said flash memory device comprising a silicon substrate, a firstelectrode of polysilicon formed on said silicon substrate with aninsulation film interposed therebetween, and a second electrode formedon said first electrode with an inter-electrode oxide film interposedtherebetween, characterized in that said inter-electrode oxide film isformed by a process comprising the steps of: depositing a polysiliconfilm on said silicon substrate as said first electrode; and exposing asurface of said polysilicon film to atomic state oxygen O* formed byexciting plasma in a mixed gas of a gas containing oxygen and an inertgas predominantly of a Kr gas by a microwave.
 21. A method offabricating a flash memory device, said flash memory device comprising asilicon substrate, a first electrode of polysilicon formed on saidsilicon substrate with an insulation film interposed therebetween, and asecond electrode formed on said first electrode with an inter-electrodenitride film, characterized in that said inter-electrode nitride film isformed by a process comprising the steps of: depositing a polysiliconfilm on said silicon substrate as said first electrode; and exposing asurface of said polysilicon film to hydrogen nitride radicals NH* formedby exciting plasma in a mixed gas of a gas containing nitrogen andhydrogen and an inert gas predominantly of a Kr gas by a microwave. 22.A method of fabricating a flash memory device, said flash memory devicecomprising a silicon substrate, a first electrode of polysilicon formedon said silicon substrate with an insulation film interposedtherebetween, and a second electrode formed on said first electrode withan inter-electrode oxynitride film interposed therebetween,characterized in that said inter-electrode oxynitride film being formedby a process comprising the steps of: depositing a polysilicon film onsaid silicon substrate as said first electrode; and converting a surfaceof said polysilicon film to a silicon oxynitride film by exposing saidpolysilicon film to plasma formed by exciting a mixed gas of an inertgas predominantly of Ar or Kr and a gas containing oxygen and nitrogenby a microwave.
 23. A method of forming a silicon oxide film,characterized by the steps of: depositing a polysilicon film on asubstrate; and forming a silicon oxide film on a surface of saidpolysilicon film by exposing the surface of said polysilicon film toatomic state oxygen O*, said atomic state oxygen O* being formed byexciting plasma in a mixed gas of a gas containing oxygen and an inertgas predominantly of a Kr gas by a microwave.
 24. A method of forming asilicon oxide film as claimed in claim 23, characterized in that saidmixed gas is a mixture of oxygen and an inert gas predominantly of a Krgas with a mixing ratio of 3% for oxygen and 97% for the inert gas. 25.A method of forming a silicon oxide film as claimed in claim 23,characterized in that said plasma has an electron density of 10¹² cm⁻³or more on said surface of said polysilicon film.
 26. A method offorming a silicon oxide film as claimed in claim 23, characterized inthat said plasma has a plasma potential of 10V or less at said surfaceof said polysilicon film.
 27. A method of forming a silicon nitridefilm, characterized by the steps of: depositing a polysilicon film on asubstrate; and forming a nitride film on a surface of said polysiliconfilm by exposing the surface of said polysilicon film to hydrogennitride radicals NH*, said hydrogen nitride radicals NH* being formed byplasma that is excited in a mixed gas of a gas containing nitrogen andhydrogen as constituent elements and an inert gas predominantly of an Argas or a Kr gas by a microwave.
 28. A method of forming a siliconnitride film as claimed in claim 27, characterized in that said gascontaining nitrogen and hydrogen is an NH₃ gas.
 29. A method of forminga silicon nitride film as claimed in claim 27, characterized in thatsaid mixed gas is a mixture of an NH₃ gas and an inert gas predominantlyof an Ar gas or a Kr gas with a mixing ration of 2% for said NH₃ gas and98% for said inert gas.
 30. A method of forming a silicon nitride filmas claimed in claim 27, characterized in that said gas containingnitrogen and hydrogen is a mixed gas of an N₂ gas and an H₂ gas.
 31. Amethod of forming a silicon nitride film as claimed in claim 27,characterized in that said plasma has an electron density of 10¹² cm⁻³or more at said surface of said polysilicon film.
 32. A method offorming a silicon nitride film as claimed in claim 27, characterized inthat said plasma has a plasma potential of 10V or less at said surfaceof said polysilicon film.
 33. A method of forming an oxynitride film,characterized by the steps of: depositing a polysilicon film on asubstrate; and converting a surface of said polysilicon film to asilicon oxynitride film by exposing said polysilicon film to plasmaformed by exciting a mixed gas of an inert gas predominantly of Ar or Krand a gas containing oxygen as a constituent element and a gascontaining nitrogen as a constituent element, by a microwave.
 34. Amethod of forming a silicon oxynitride film as claimed in claim 33,characterized in that said gas containing nitrogen is an NH₃ gas.
 35. Amethod of forming a silicon oxynitride film as claimed in claim 33,characterized in that said mixed gas is a mixture of an inert gaspredominantly of Ar or Kr and an oxygen gas and an NH₃ gas with a mixingratio of 96.5% for said inert gas and 3% for said oxygen gas and 0.5%for said NH₃ gas.
 36. A method of forming a silicon oxynitride film asclaimed in claim 33, characterized in that said gas containing nitrogenis a mixed gas of an N₂ gas and an H₂ gas.
 37. A method of forming asilicon oxynitride film as claimed in claim 33, characterized in thatsaid plasma has an electron density of 10¹² cm⁻³ or more at said surfaceof said polysilicon film.
 38. A method of forming a silicon oxynitridefilm as claimed in claim 33, characterized in that said plasma has aplasma potential of 10V or less at said surface of said polysiliconfilm.
 39. A method of forming a silicon oxide film on a polysiliconfilm, characterized by the steps of: forming plasma containing thereinatomic state oxygen O* in a processing vessel of a microwave processingapparatus, said microwave processing apparatus including, in addition tosaid processing vessel, a shower plate provided in a part of saidprocessing vessel so as to extend parallel with a substrate to beprocessed, said shower place including a number of apertures forsupplying a plasma gas toward said substrate to be processed, and amicrowave radiation antenna provided such that said microwave radiationantenna emits a microwave into said processing vessel through saidshower plate, said plasma being formed by supplying an inert gaspredominantly of Kr and a gas containing oxygen into said processingvessel via said shower plate, and by supplying a microwave into saidprocessing vessel from said microwave radiation antenna through saidshower plate; and oxidizing, in said processing vessel, a surface ofsaid polysilicon film formed on said substrate by said plasma, to formsaid silicon oxide film.
 40. A method of forming a silicon oxide film asclaimed in claim 39, characterized in that said plasma has an electrondensity of 10¹² cm⁻³ or more at said surface of said polysilicon film.41. A method of forming a silicon oxide film as claimed in claim 39,characterized in that said plasma has a plasma potential of 10V or lessat said surface of said polysilicon film.
 42. A method of forming asilicon nitride film on a polysilicon film, characterized by the stepsof: forming plasma containing therein hydrogen nitride radicals NH* in aprocessing vessel of a microwave processing apparatus, said microwaveprocessing apparatus including, in addition to said processing vessel, ashower plate provided in a part of said processing vessel so as toextend parallel with a substrate to be processed, said shower placeincluding a number of apertures for supplying a plasma gas toward saidsubstrate to be processed, and a microwave radiation antenna providedsuch that said microwave radiation antenna emits a microwave into saidprocessing vessel through said shower plate, said plasma being formed bysupplying an inert gas predominantly of Ar or Kr and a gas containingnitrogen and hydrogen into said processing vessel via said shower plate,and by supplying a microwave into said processing vessel from saidmicrowave radiation antenna through said shower plate; and nitriding, insaid processing vessel, a surface of said polysilicon film formed onsaid substrate by said plasma, to form said silicon nitride film.
 43. Amethod of forming a silicon nitride film as claimed in claim 42,characterized in that said gas containing nitrogen and hydrogen is anNH₃ gas.
 44. A method of forming a silicon nitride film as claimed inclaim 42, characterized in that said gas containing nitrogen andhydrogen is a mixed gas of an N₂ gas and an H₂ gas.
 45. A method offorming a silicon nitride film as claimed in claim 42, characterized inthat said plasma has an electron density of 10¹² cm⁻³ or more at saidsurface of said polysilicon film.
 46. A method of forming a siliconnitride film as claimed in claim 42, characterized in that said plasmahas a plasma potential of 10V or less at said surface of saidpolysilicon film.
 47. A method of forming a silicon oxynitride film on apolysilicon film, characterized by the steps of: forming plasmacontaining therein atomic state oxygen O* and hydrogen nitride radicalsNH* in a processing vessel of a microwave processing apparatus, saidmicrowave processing apparatus including, in addition to said processingvessel, a shower plate provided in a part of said processing vessel soas to extend parallel with a substrate to be processed, said showerplace including a number of apertures for supplying a plasma gas towardsaid substrate to be processed, and a microwave radiation antennaprovided such that said microwave radiation antenna emits a microwaveinto said processing vessel through said shower plate, said plasma beingformed by supplying an inert gas predominantly of Ar or Kr and a gascontaining oxygen as a constituent element and a gas containing nitrogenas a constituent element into said processing vessel via said showerplate, and by supplying a microwave into said processing vessel fromsaid microwave radiation antenna through said shower plate; andoxynitriding, in said processing vessel, a surface of said polysiliconfilm formed on said substrate by said plasma, to form said siliconoxynitride film.
 48. A method of forming a silicon oxynitride film asclaimed in claim 47, characterized in that said gas containing nitrogenand hydrogen is an NH₃ gas.
 49. A method of forming a silicon oxynitridefilm as claimed in claim 47, characterized in that said gas containingnitrogen and hydrogen is a mixed gas of an N₂ gas and an H₂ gas.
 50. Amethod of forming a silicon oxynitride film as claimed in claim 47,characterized in that said plasma has an electron density of 10¹² cm⁻³or more at said surface of said polysilicon film.
 51. A method offorming a silicon oxynitride film as claimed in claim 42, characterizedin that said plasma has a plasma potential of 10V or less at saidsurface of said polysilicon film.